Peripheral Glucagon-like Peptide-1 (GLP-1) and Satiation

Physiology & Behavior xxx (2011) xxx–xxx

PHB-09381; No of Pages 6

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Peripheral Glucagon-like Peptide-1 (GLP-1) and Satiation

Mukesh Punjabi, Myrtha Arnold, Nori Geary, Wolfgang Langhans, Gustavo Pacheco-López ⁎Physiology and Behaviour Laboratory, Institute of Food, Nutrition and Health, ETH Zurich, 8603 Schwerzenbach, Switzerland

⁎ Corresponding author at: PhysiologyandBehaviour Lab16, 8603 Schwerzenbach, Switzerland. Tel.: +41 44 655 7

E-mail address: [email protected] (G. Pache

0031-9384/$ – see front matter © 2011 Elsevier Inc. Aldoi:10.1016/j.physbeh.2011.02.038

Please cite this article as: Punjabi M, et al,physbeh.2011.02.038

a b s t r a c t

Article history:Received 21 February 2011Accepted 23 February 2011Available online xxxx

Keywords:Exendin-4Food intakeEatingVagusHindbrainHepatic portal veinIncretinMeal size

Peripheral GLP-1 is produced by post-translational processing of pro-glucagon in enteroendocrine L-cellsand is released in response to luminal nutrient (primarily carbohydrate and fat) stimulation. GLP-1 is wellknown for its potent insulinotropic and gluco-regulatory effects. GLP-1 receptors (GLP-1R) are expressed inthe periphery and in several brain areas that are implicated in the control of eating. Both central andperipheral administration of GLP-1 have been shown to reduce food intake. Unresolved, however, iswhether these effects reflect functions of endogenous GLP-1. Data collected in our laboratory indicate thatin chow-fed rats: 1) Remotely controlled, intra-meal intravenous (IV) or intraperitoneal (IP) GLP-1infusions selectively reduce meal size; 2) hindbrain GLP-1R activation is involved in the eating-inhibitoryeffect of IV infused GLP-1, whereas intact abdominal vagal afferents are necessary for the eating-inhibitoryeffect of IP, but not IV, infused GLP-1; 3) GLP-1 degradation in the liver prevents a systemic increase inendogenous GLP-1 during normal chowmeals in rats; and 4) peripheral or hindbrain GLP-1R antagonism byexendin-9 does not affect spontaneous eating. Also, although our data indicate that peripheral GLP-1 can actin two different sites to inhibit eating, they argue against a role of systemic increases in endogenous GLP-1in satiation in chow-fed rats. Therefore, further studies should examine whether a local paracrine action ofGLP-1 in the intestine or and endocrine action in the hepatic-portal area is physiologically relevant forsatiation.

oratory, ETHZurich, Schorenstr.420; fax: +41 44 655 7206.co-López).

l rights reserved.

Peripheral Glucagon-like Peptide-1 (GLP-1)

© 2011 Elsevier Inc. All rights reserved.

1. Introduction

Glucagon-like-peptide (7–36) amide (GLP-1) is the product of post-translational processing of pro-glucagon in the gut and the brain [1]. Inthe brain, GLP-1 is expressed in some neurons of the nucleus tractussolittarii (NTS) [2,3]. In the gut, GLP-1 is released primarily fromenteroendocrine L-cells located in the distal jejunum and ileum [1].Luminal carbohydrates and fats are potent stimuli for GLP-1 secretion,and these effects appear to be mediated in part by taste receptorsexpressed on enteroendocrine cells [4] (Fig. 1). Recently GLP-1 has alsobeen identified in sweet- and umami-taste receptor cells in the oralcavity [5], suggesting that GLP-1 signaling is involved in taste. GLP-1receptors (GLP-1R) arewidely distributed in the brain and in peripheralorgans such as the pancreatic islets and the whole gastrointestinal (GI)tract [6,7]. In the brain, GLP-1R are found in several hindbrain andforebrain areas [8], including areas that are implicated in the control offood intake and energy balance, such as the area postrema (AP), NTS,hypothalamus and amygdala [3]. GLP-1 has several physiologicaleffects: it acts as a strong incretin, i.e., stimulates glucose-inducedinsulin release [9], it inhibits glucagon release [1], and it inhibits gastricemptying, i.e., contributes to the ileal brake mechanism [10]. Further

potential physiological effects of GLP-1 include an influence on learningand memory [11], neuroprotection [12,13], and the inhibition of eatingand drinking [14,15]. Administration of GLP-1 or of potent, long-actingnatural or synthetic GLP-1 analogs, such as exendin-4 (Ex-4) orliraglutide, inhibits eating in many species, including humans [16–22].Ex-4 is a naturally occurring peptide resistant to rapid enzymaticdegradationbydipeptidyl peptidase 4 (DPP-4) [23]. The syntheticGLP-1analog liraglutide also resists enzymatic degradation and, because itbinds to albumin in the plasma, delays renal excretion [23]. Chronicadministrations of GLP-1 or its analogs have effectively reduced weightand improved glucose metabolism in overweight individuals [18,24],suggesting GLP-1 may offer an effective therapeutic option foroverweight and type II diabetes [25].

Endogenous intestinal GLP-1 has been implicated in mealtermination (satiation) because: 1) exogenous GLP-1 inhibited eatingprimarily by reducing meal size [26,27] and 2) intraperitoneal (IP)injection of the specific GLP-1R antagonist exendin (9–39)(Ex-9)stimulated eating under some conditions [26], indicating thatsatiation can be delayedwhen endogenous GLP-1 signaling is blocked.In other studies, however, Ex-9 administration failed to stimulateeating, suggesting that endogenous GLP-1 is not required for thecontrol of meal size under all conditions. Thus, further research iswarranted to identify the situations under which endogenous GLP-1 isphysiologically relevant for satiation. Furthermore, the site andmechanism of GLP-1's eating-inhibitory action, including the intra-cellular signaling cascades involved, are still largely unknown.

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After secretion, GLP-1 is rapidly degraded into GLP-1 (9–36) byDPP-IV [28]. As a result, the biological half-life of GLP-1 is less than5 min [29]. If prandial secretion is maintained and exceeds degrada-tion, endogenous GLP-1 may still have an endocrine satiating effect bydirect central nervous system action. In addition, such a route ofaction may be particularly relevant for pharmacological interventionsthat lead to sustained, and substantial increases in circulating levels ofnative GLP-1 or GLP-1R agonists. It is also relevant to mention thatparacrine effects of GLP-1 in rats may be endocrine effects in humans,as is the case for the satiating effect of cholecystokinin (CCK). Finally,gut hormones can also signal the brain through an endocrine orparacrine effect on afferent nerves, with the vagus being theprominent neural route of gut-brain signaling [30,31]. Whether ornot endogenous peripheral GLP-1 induces satiation through such anendocrine or paracrine action is still unresolved. This review brieflysummarizes what is known about these open questions, capitalizingon some experiments that we recently did to address them.

2. Endocrine effects of GLP-1

The insulinotropic effect of GLP-1 is mediated in part by GLP-1R onhepatic branch vagal afferents terminating in the wall of the hepaticportal vein close to the liver [32]. Activation of these receptors triggersa vago-vagal reflex that increases pancreatic vagal-efferent activityand stimulates insulin release [33]. Therefore, it appeared logical totest whether a similar mechanism is involved in the eating-inhibitoryeffect of peripheral GLP-1. In an attempt to mimic the meal-inducedrelease of endogenous GLP-1 from the small intestinal L-cells into thehepatic portal vein as closely as possible, we equipped rats withchronic hepatic portal vein catheters and infused various doses ofGLP-1 during the first spontaneous nocturnal meal. The animals werecontinuously monitored via infra-red video cameras, and theinfusions were triggered by remote control from an adjoining roomto prevent disturbing them. Under these conditions, hepatic portalvein infusions of GLP-1 reduced the size and duration of the ongoingmeal but did not affect the subsequent inter-meal interval or thesize or duration of the second meal [27]. Furthermore, intra-mealhepatic portal vein and vena cava (i.e., systemic) infusions of GLP-1(1 nmol/kg body weight [BW]) produced similar effects on eating,suggesting that the satiating effect of circulating GLP-1 is notmediated by GLP-1R located in the hepatic portal system or liver.Interestingly, although hepatic portal vein infusion of Ex-4 under thesame conditions reduced the sizes of both the first and the secondmeals, it did not affect the duration of the intervening inter-mealinterval, suggesting that GLP-1R activation has a selective effect onsatiation. Whether the briefer and smaller increases in plasma GLP-1levels produced by GLP-1 infusions and the more prolonged andgreater increases in GLP-1R activation produced by Ex-4 reduce foodintake by acting at the same site(s) or whether Ex-4 recruitsadditional receptors for its extended effect remains to be investigated.In addition, while all these findings are consistent with a possiblephysiological endocrine satiating effect of endogenous GLP-1, our datato date do not support a role of GLP-1 in the graded satiating effect offood because we have not found a clearly dose-related satiating effectof exogenous GLP-1 [27].

We also examinedwhether the eating-inhibitory effect of circulatingGLP-1 is mediated by abdominal vagal afferents. Rats underwent sub-diaphragmatic vagal deafferentation (SDA) [34] or sham surgery andwere equipped with hepatic portal vein catheters. SDA produces themost specific and complete lesionof abdominal vagal afferents. That is, itlesions both vagal A- and C-fibers, whereas capsaicin lesions onlyunmyelinatedvagalC-fibers and lesionsnon-vagal aswell as vagalfibers[35]. In addition, SDA leaves about 50% of the vagal efferents intact[34,36], which reduces the adverse side effects related to impaired GImotility and secretion that are produced by complete subdiaphragmaticvagotomy. Intra-meal hepatic portal vein infusions of GLP-1 reduced

Please cite this article as: Punjabi M, et al, Peripheral Glucagon-like Pephysbeh.2011.02.038

meal size and duration similarly in SDA and sham-operated rats [27]. Inaddition, Ex-4 reduced first (Fig. 2) and second meal size (data notshown)without a significant effect on othermeal parameters. Ex-4 alsoproduced similar 2 and 4 h decreases in cumulative food intake in SDAand sham-operated rats (Fig. 3). Overall, these data indicate thatabdominal vagal afferents are not necessary for the eating-inhibitoryeffect of circulating GLP-1 or Ex-4.

In order to identify the neural substrates that may be involved inmediating the satiating effect of circulating GLP-1, we assessed c-Fosexpression in some hindbrain (area postrema [AP], NTS) and forebrainstructures (central nucleus of the amygdala [CeA], hypothalamicparaventricular and arcuate nuclei [PVN and Arc]) implicated in thecontrol of eating. We found that hepatic portal vein GLP-1 infusion, at adose (1 nmol/kg BW) that reliably inhibited eating, activated the NTS,AP, and CeA, suggesting that these brain areas participate in theendocrine eating-inhibitory effect of GLP-1 [37]. One previous studyusing femoral vein infusion of a lower dose of native GLP-1(1 μg=0.24 nmol/kg BW) failed to detect a significant increase in c-Fos expression in the brainstem [38], whereas another study usingfemoral vein infusions of Ex-4 [39] found awider-spread activation thanwe did. Because of themuch longer biological half-life of Ex-4, however,these results cannot directly be compared to ours.

In addition to causing satiation, GLP-1 may inhibit eating non-specifically by inducing malaise. GLP-1 can trigger a conditioned tasteaversion and pica (kaolin intake) in rodents [40,41]. On the otherhand, behavioral observations in macaques did not reveal any overtsigns of malaise, such as decreased alertness, drooling, or vomiting,in response to GLP-1 [16]. Under which conditions GLP-1 producessatiation or malaise remains unclear. GLP-1R in the CeA appear tomediate some of the response to peripheral illness [42]. This isbecause 1) intra-amygdala administration of GLP-1, but not of theinactive analog, GLP-1 (9–39), produced a strong conditioned tasteaversion, similar to those produced by visceral malaise or foodpoisoning, and 2) intra-amygdala administration of Ex-9 preventedtaste aversion learning in response to IP injections of the toxin lithiumchloride [42]. Importantly, intra-amygdala injection of GLP-1 did notitself reduce food intake, suggesting that this mechanism is not anecessary component of the eating-inhibitory effect of endocrineGLP-1. Presumably, GLP-1 projections from the NTS are involved inthis receptor activation. At the same time, however, our c-Fos data areconsistent with the possibility that the CeA may also be involved inmediating the eating-inhibitory effect of circulating GLP-1. Whetheror not this increase in CeA c-Fos occurred in GLP-1R-expressingneurons or was related to an aversive component of GLP-1's eating-inhibitory effect under our conditions requires further research.

We recently found that infusion of Ex-9 (10 μg/rat) into the 4thcerebral ventricle blocked the eating-inhibitory effect of hepatic portalvein GLP-1 infusion in rats, indicating that GLP-1R that are accessiblefrom the 4th ventricle (e.g. NTS and AP) are involved in mediating thiseffect [43]. Also, preliminary findings of ours indicate that AP lesionsblock the eating-inhibitory effect of hepatic portal vein GLP-1infusions, suggesting that the AP is involved as well (Punjabi et al., inpreparation). IP injection of (2.0 μg/kg BW)Ex-4was recently reportedto reduce food intake in AP-lesioned rats after IP injection [44], whichat first glance seems to contradict the idea that the AP mediates theeating-inhibitory effect of circulating GLP-1. It is possible, however,that the greater and more prolonged GLP-1R activation produced byEx-4 may have recruited redundant neural circuits to inhibit eating.It may be relevant in this connection that it was not determinedwhether Ex-4 elicited a conditioned taste aversion after AP lesions.

3. GLP-1R mechanisms

GLP-1R are G-protein-coupled and activate diverse intracellularsignaling pathways involving cyclic adenosine monophosphate,protein kinase A (PKA), phospholipase C, phosphatidylinositol-3

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kinase, protein kinase C, p44/42 mitogen-activated protein kinase(p44/42 MAPK; also known as extracellular signal-regulated kinase[ERK]-1/2), and intracellular Ca2+ [45,46]. The neuronal intracellularsignaling pathways mediating the eating-inhibitory effect of GLP-1are still poorly characterized. We recently started to address thisquestion using strategies based on the mediation of the satiatingeffect of CCK and on the intracellular signaling pathway activated byGLP-1 in pancreatic β-cells and in neurons. CCK increases activity ofthe ERK signaling cascade in NTS neurons, and inhibiting thispathwaywith an ERK kinase (MEK) inhibitor significantly attenuatedthe eating-inhibitory effect of IP CCK in rats [47,48]. GLP-1Ractivation also leads to ERK phosphorylation through a MEK-dependent pathway in pancreatic β-cells [49]. Furthermore, ERKphosphorylation was significantly increased in the hippocampus20 min after intranasal administration of the short GLP-1 analog, [Ser(2)] exendin(1–9), and the enhancement of associative learning bythis GLP-1 analogwas completely blocked by intracerebroventricularinfusion of aMEK inhibitor [11]. In linewith this, it has been proposedthat hindbrain GLP-1R activation leads to an increase in ERK activity[14]. Preliminary data from our laboratory in fact suggest that hepaticportal vein GLP-1 infusion increases phosphorylation of ERK 2, butnot ERK 1, in the NTS (Punjabi et al., unpublished). Whetheractivation of the ERK 1/2 signaling pathway is involved in thesatiating effect of circulating GLP-1, however, remains to beinvestigated.

4. Prandial increases in circulating GLP-1 levels

Increases in circulating endogenous GLP-1 levels in laboratoryanimals have been reported only in response to intragastric [50,51]or intraintestinal [52] administration of liquid diets or nutrientsolutions in anesthetized animals, i.e., in situations that do notnecessarily mimic ingestion of a solid, mixed-nutrient meal inundisturbed, awake animals. To determine whether meals doincrease circulating GLP-1 levels, we equipped rats with hepaticportal vein and vena cava catheters, took blood samples in parallelfrom both blood vessels during and after spontaneous nocturnalchowmeals (fixed in size to 3 g), and assayed active GLP-1.We foundthat GLP-1 increased during the meal in the hepatic portal vein, butnot in the vena cava [53]. Hepatic portal vein plasma concentration ofGLP-1 increased rapidly, reached a peak (∼17 pM) 6 min after mealonset, and returned to baseline values (5–7 pM) by 20 min aftermealonset. Vena cava GLP-1 levels did not increase at any time. When1 nmol/kg BW GLP-1 was infused into the hepatic portal vein, activeGLP-1 in the vena cava increased to∼360 pM immediately after theend of infusion and returned to baseline levels 20 min later. Theprandial increase in endogenous GLP-1 that we observed was muchsmaller than in previous tests of intragastric or intraintestinalnutrient administration [50,51]. We presume that this discrepancyis due to the different test conditions for the reasons described above.Also, we measured active GLP-1 [i.e. GLP-1 (7–36 amide) and GLP-1(7–37)], whereas D'Alessio et al. [50], for instance, determined totalGLP-1 levels using an antiserum that reacts with an epitope includingthe COOH-terminal amide and therefore also recognizes the inactiveprecursor GLP-1 (1–36) and metabolite GLP-1 (9–36). Our failure tofind a significant prandial increase of active GLP-1 in the vena cava isconsistent with other reports. For instance, a 5 ml meal of Ensure(Abbott Laboratories) failed to increase systemic active GLP-1 levelsin rats [54], and intragastric intubation of glucose and corn oil causeda much smaller increase in GLP-1 concentration in the inferior venacava than in the hepatic portal vein [51]. Meal-related changes inGLP-1 have been reported in humans (e.g. [55,56]), but these studiesdid not include GLP-1 measurements in the HPV. Some of thesestudies also measured total instead of active GLP-1. Although totalGLP-1 is a reliable index of secreted hormone, it is not a useful

Please cite this article as: Punjabi M, et al, Peripheral Glucagon-like Pephysbeh.2011.02.038

measure of the amount of active hormone present because GLP-1degradation rates exceed clearance rates [57].

All in all, our findings suggest that during a nocturnal chow mealin rats, GLP-1 released from intestinal L-cells may have local,paracrine effects or endocrine effects in the hepatic portal vein orliver, but not systemic endocrine effects (Fig. 1). Furthermore,1 nmol/kg BW GLP-1 is only about twice the threshold dose for asatiating effect under these conditions [27], but increased systemicGLP-1 levels almost 25-fold the prandial physiological level in thehepatic portal vein. Our findings therefore also suggest that theinhibition of eating after intravenous infusion of exogenous GLP-1under these conditions is a pharmacological effect. Thus, our dataargue against a role of systemic increases in endogenous GLP-1 insatiation, at least under our conditions.

5. Paracrine effects of GLP-1

Because meals do not appear to increase systemic GLP-1 levels, wehave also tested the effects of IP infusionsof GLP-1 on eating. IP infusionsmay be better suited than hepatic portal vein infusions to target thevagal afferents terminating in thewall of the small intestine and, hence,address the possible role of a paracrine effect of endogenous GLP-1 insatiation. Remotely controlled, intra-meal IP GLP-1 infusions weretested under the same conditions as described above for hepatic portalvein infusions [58]. Similar to its effect after hepatic portal vein infusion,IP GLP-1 selectively reduced meal size and duration, although thethreshold dose (10 nmol/kg BW) for a reliable reduction of meal sizeand duration was about ten times higher than for hepatic portal veininfusion [58]. In contrast to its failure to block the eating-inhibitoryeffect of GLP-1 after hepatic portal vein infusion, however, SDA blockedthe satiating effect of IP infusion of 10 nmol/kg BW GLP-1 [58]. Thus,vagal afferent signaling is necessary for the full eating-inhibitory effectof IP, but not hepatic portal vein, GLP-1 [53,58]. Whether or not theeating-inhibitory effect of IP Ex-4 also requires intact abdominal vagalafferents remains to be examined. The acute hyperglycemia induced byIP Ex-4 administrationwas recently reported to be independent of vagalafferent signaling [59], suggesting that IP administered Ex-4 can reachnon-vagal GLP-1R.

The interstitial fluid in the lamina propria of the intestinal mucosaenters the lymphatic ducts by bulk flow, so that the intestinal lymphreflects the composition of the interstitial fluid in the lamina propriaof the mucosa fairly well [60,61]. Vagal afferents also terminate in thelamina propria (Fig. 1)[62,63]. Furthermore, IP injected peptides alsoreach the lamina propria and, thus, the intestinal lymph [50,64,65]. Inaddition, intragastric nutrient infusions increase the GLP-1 concen-tration in intestinal lymph (i.e. chyle) more than in the hepatic portalblood [50]. Finally, GLP-1 has been shown to activate gastric vagalafferents in whole nerve recordings [66], suggesting a similar effect onintestinal vagal afferents. These facts together with our SDA findingsdescribed above are consistent with the idea that IP GLP-1 may act onintestinal vagal afferents to inhibit eating, i.e., that IP GLP-1administration mimics a paracrine satiating effect of endogenousGLP-1 during meals.

To more directly examine the possible physiological relevance ofendogenous intestinal GLP-1, we IP infused Ex-9 [58]. Although Ex-9clearly antagonizes the incretin effect of endogenous GLP-1 inhumans [67] and rats [68,69], currently available reports on theeffects of peripheral Ex-9 on eating are inconsistent. IP Ex-9increased food intake only under some of the conditions tested(i.e. a moderate dose [30 μg/kg=8.9 nmol/kg BW] given in themiddle of the light phase or a higher dose [100 μg/kg=29.7 nmol/kgBW] given at 1 h into the dark phase [26]). Furthermore, othersfound that hepatic portal vein or jugular vein infusion of anotherGLP-1 receptor antagonist (desHis1,Glu8exendin-4) did not increasefood intake in rats [70]. Under our conditions, intra-meal infusion of30 nmol/kg BW Ex-9 did not increase the size of the first or second

ptide-1 (GLP-1) and Satiation, Physiol Behav (2011), doi:10.1016/j.

Fig. 1. Some possible pathways for the eating-inhibitory effect of peripheral GLP-1: After nutrients in the chyme induces GLP-1 release by a "taste" receptor signaling mechanismpresent in enteroendocrine L-cells, (1) GLP-1 could be detected by a paracrine action on sub-mucosal vagal afferent nerves expressing GLP-1 receptors [85] with subsequent afferentsignaling to brainstem nuclei, such as the nucleus tractus solitarii (NTS). An endocrine action, requiring that enough peripheral GLP-1 escapes inactivation by dipeptidyl peptidase-IV(DPP-IV) to reach the circulation. In this case, (2) detection by peripheral vagal afferents could still occur in the hepatic portal region [86] or on the lymphatic system, since highpostprandial GLP-1 levels has been detected on the chyle [50]. Alternatively, (3) GLP-1 that survives liver passage may act directly on the brain through circumventricular organssuch as the area postrema (AP) [44]. It has been proposed that hindbrain GLP-1 receptor activation leads to an increase in extracellular signal-regulated kinase (ERK) 1/2phosphorylation [14], and preliminary data from our laboratory suggest that phosphorylation of ERK2 is increased in the NTS by hepatic portal vein GLP-1 infusions. GLP-1 neuralactions, however, could be both pre- or post-synaptically.

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nocturnal meal [58], although the same dose effectively blocked theeating-inhibitory effect of IP infusion of 10 nmol/kg BW GLP-1,indicating that the Ex-9 could indeed reach the receptors thatmediate the eating-inhibitory effect of exogenous GLP-1 in asufficiently high concentration and early enough to antagonize thiseffect [58]. This suggests that endogenous GLP-1 is not necessary forsatiation under these conditions at dark onset [26]. It is also possiblethat endogenous GLP-1 acts more rapidly than did our IP GLP-1infusions, so that that the Ex-9 dose we used reached the criticalGLP-1R too late or in too small a concentration to influence the effectof endogenous GLP-1. This is particularly likely if endogenous GLP-1inhibits eating by a paracrine effect on vagal afferents terminating inthe small intestinal mucosa, i.e. very close to the site of its release

0

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Fig. 2. Subdiaphragmatic vagal deafferentation (SDA) does not affect the satiating effectof hepatic portal vein infusion of the long-acting GLP-1 agonist exendin-4 (Ex-4,0.25 nmol/kg body weight). Adult male rats (SDA, n=9, Sham, n=7) were fooddeprived for the last 3 h of the light phase. Infusions(0.25 ml/min) of Ex-4 or vehicle(phosphate buffered saline+1% bovine serum albumin) began 2 min after meal onset.*Pb0.05 vs. Veh, paired t-test.

Please cite this article as: Punjabi M, et al, Peripheral Glucagon-like Pephysbeh.2011.02.038

[50]. Other differences in experimental design (e.g. food restrictiontime, rat strain, injection vs. infusion by chronic catheter, etc.) mayhave contributed to the different outcomes as well. Further studiesare necessary to clarify these issues.

Finally, GLP-1 has been shown to synergize with glucose tostimulate insulin release from the pancreas, and this effect appears tobe mediated by vagal afferents originating in the hepatic portal area[71]. The glucose transporter GLUT2 and the GLP-1 receptor are keycomponents required for glucose-dependent insulin secretion [72]and for the proper function of the hepatic portal vein glucose sensors[73]. While the satiating effect of exogenous GLP-1 does not appear tobe mediated by these hepatic portal vein receptors (see above), itcould well be that a similar synergistic interaction occurs at the vagal

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Fig. 3. Subdiaphragmatic vagal deafferentation (SDA) does not affect the reduction incumulative food intake in response to hepatic portal vein infusion of Ex-4 (0.25 nmol/kg body weight). See legend of Fig. 2 for further details.*Pb0.05 vs. Veh, paired t-test.

ptide-1 (GLP-1) and Satiation, Physiol Behav (2011), doi:10.1016/j.

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afferents GLP-1R in the intestinal wall. We therefore started toexamine whether IP GLP-1 may also interact with glucose to inhibiteating. Preliminary data from these experiments suggest that sub-threshold doses of GLP-1 and glucose can combine to reduce mealsize after intra-meal, IP infusion (Rüttimann et al., unpublished). Thisraises the possibility that GLP-1 may generally require glucose toactivate vagal afferents, which would extend the concept of the gluco-incretin functional unit model [72–74].

6. Perspectives

Spurred by the limited effectiveness of the few currently availabletreatment options for obesity and by the multiple side effects andproblems frequently encountered with drugs acting directly in thebrain, basic and clinical obesity researchers have begun to shift theirattention from the brain to the GI tract and in particular to thepotential of GI peptides in the control of energy homeostasis. GIpeptides link intestinal nutrient sensing to systemic effects mediatedby neuroendocrine or neural reflexes, and many GI peptides havebeen found to reduce food intake. As mentioned above, GLP-1 and itsanalogs are particularly promising for the pharmacotherapy of obesityand type II diabetes because GLP-1 is still effective in obese individuals[16–22,75] and has potent beneficial effects on metabolism [1]. Also,recent evidence from intestinal nutrient infusion studies in laboratoryanimal models as well as from pharmacological antagonist studies inhumans indicate that the dramatic obesity-curbing effect of bariatricsurgery is at least in part related to the enhanced release of GLP-1 andother, primarily distal small intestinal, eating-inhibitory GI peptides[54,76]. Moreover, previous concerns about tolerance to the eating-inhibitory effects of GI peptides with chronic administration inanimal models and, hence, about their therapeutic potential, may beunwarranted. Thus, CCK administered just prior to each meal in obeseZucker and dietary-induced obese rats adapted to three scheduledmeals/day (a feeding paradigm that is presumably more relevant forhuman eating than continuous ad libitum access to food) persistentlyreduced meal size and body weight over several weeks of treatment[77,78]. In addition, chronic administration of GLP-1 analogs resultedin significant weight loss in obese humans [79], implying that GLP-1receptor activation preserves its effectiveness with chronic use andthat this is an effective and promising strategy that deserves furtherexploration [1]. The apparent synergistic effects of GLP-1 and itssynthetic analogs with other peptides on food intake, insulin releaseand glucose utilization support the idea of an increased therapeuticpotential for "cocktails" of GLP-1 and other agents. Such peptidecocktails might combine GLP-1 or its analogs with leptin or amylin,combinations which have been shown to produce synergistic effectson eating and body weight [80,81]. Combination of GLP-1 withanother peptide in a chimeric molecule represents also a promisingapproach. Such a molecule combining GLP-1 and pancreatic glucagon(which was identified as the first physiologically relevant peripheralsatiating peptide almost 30 years ago [82]), produced dramatic effectson body weight in mice [83]. Another exciting area of new research isthe discovery of GLP-1 preparations that are rapidly absorbed andprotected from intestinal enzymatic breakdown and can therefore beadministered orally [22]. Last but not least, the fact that L-cells –

similar to other enteroendocrine cells - express some of the same"taste" receptors as the taste cells in the oral cavity [84] and releasephysiologically relevant amounts of GLP-1 in response to stimulationby tastants raises further new opportunities. For example, combina-tions of in-vitro models of the GI epithelium, in-vivo animal modelsand food technology may lead to successful targeting of the intestinalendocrine cells to exploit novel therapeutic approaches to themodulation of GI peptide release to influence eating and metabolism.

All these discoveries suggest that the sensing and secretoryfunctions of the wall of the GI tract represent a major, heretoforeunderappreciated regulatory interface in the control of eating and

Please cite this article as: Punjabi M, et al, Peripheral Glucagon-like Pephysbeh.2011.02.038

energy balance, and a viable target for pharmacological approaches tocurb obesity and metabolic disease. Although a number of mecha-nisms have already been implicated in these actions, their precisemediating mechanisms in the gut and brain as well as their truephysiological relevance remain unclear. To eventually identify themost promising strategy – or combination of approaches – to fightobesity and type II diabetes, it is critically important to thoroughlycharacterize the site(s) and mechanism(s) of action of the endoge-nous peptides or their analogs involved. We hope that this reviewsucceeded to briefly summarize some key features of currentknowledge about intestinal GLP-1 and to identify gaps in knowledgewere further research is required.

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